I was thinking more to the original use of the word stout. I don't think the Irish Stouts we know where brewed before the term started to be use.
This is actually true. When we think of stout, we tend to think of Guinness, which the Guinness Draught we all know and love wasn't released until 1959.
I agree with most on here that the difference is Roasted Barley...sometimes. I have been to the Guinness Storehouse twice now and the following is the story they give.
Back in the 19th century, the water in Ireland was undrinkable. The dockworkers were unable to drink water throughout the day and some even died of dehydration. Arthur Guinness started brewing a dark session beer that could be drank throughout the day. He called this beer a porter because it was made for the dockworkers at port to drink all day while they worked and they would not get drunk.
This is the first beer brewed by Arthur Guinness, the Dublin Porter. He routed water coming off the Wicklow Mountains into the brewery and they use that water to this day.
As for the stout, they decided to make a drier, lighter, dark beer and have a strong roasted character.
The Dublin Porter is light-medium bodied and low ABV, just like the Guinness Draught, but is a lot sweeter and has no roasted character.
The Guinness Draught is light, very dry, and uses ~600 SRM roasted barley to brew the beer. It is VERY heavily roasted and very dry.
When you think Porter, think sweet and chocolate with a coffee flavor character (not a char, roasty character at all)
When you think of a Stout, think Dry, Dark Roast Coffee. It really is just a harsher, not sweet, version of a porter.
This is where the term "Stout Porter" came from back in the day. They wanted something stronger but close to the flavor. Hence, Stout was born.
I am by no means an authority, this is just what I learned from Guinness and my own opinions on the differences of the style. I hope this helps!